1. Field of the Invention
The present invention relates to an electrostatic chuck used to attractively fix articles such as semiconductor wafers by electrostatic force.
2. Description of Related Art
Electrostatic chucks provided with an internal electrode between a substrate and an insulating layer (dielectric layer) are used as wafer fixing jigs when semiconductor wafers are treated by plasma etching, CVD, ion plating or the like.
The electrostatic chucks are required to perform such that a great electrostatic force is produced during the application of voltage so that attracted articles are prevented from falling or moving and after elimination of the applied voltage the electrostatic attractive force is immediately made small so that the attracted articles can be displaced with ease.
Means for increasing electrostatic attractive force include, e.g., means in which the insulating layer is made to have a large dielectric constant (Japanese Patent Publications No. 60-59104 and No. 62-19060), means in which the thickness of the insulating layer is controlled (Japanese Patent Laid-open Publication No. 57-64950) and means in which the insulating layer is made to have a volume resistivity in a given range (Japanese Patent Publication No. 61-14660 and Japanese Patent Laid-open Publication No. 2-22166). Means for making the displacement of attracted articles easy include, e.g., means in which helium gas is blown between the chuck surface and an attracted article (Japanese Utility Model Laid-open Publication No. 2-120831) and means for applying a voltage with polarity opposite to the voltage applied when wafers are attracted (Japanese Patent Publication No. 2-63304).
The above conventional means for increasing electrostatic attractive force take note of only the insulating layer, where residual attractive force tends to become larger with an increase in electrostatic attractive force. With the conventional means, however, it takes as long as at least 60 seconds before the residual attractive force attenuates and the attracted article can be displaced with ease. Thus, with the conventional means it is not possible to have a response in which attracted articles may be displaced immediately after treatment. Hence, there is a disadvantage that an additional device or an additional operation other than those normally used must be added in order to make the displacement of attracted articles easy. In particular, there is a problem when the known electrostatic chucks are used in an environment of low temperature.
One reason for the above limitations of known electrostatic chucks is that the materials constituting the insulating layer (dielectric layer) of conventional electrostatic chucks do not allow the volume resistivity of the insulating layer to be arbitrarily controllable over a wide range. In this regard, it has been proposed to add titania to alumina as a transition metal oxide. In this instance, composite oxide grains deposited between alumina grains increase with an increase in the amount of titania added, resulting in a remarkable dependence of electrical conductivity on electric field intensity. Namely, the volume resistivity abruptly decreases at a certain electric field intensity that forms a border, making it impossible to maintain a given electrostatic performance.